For modern commercial aircraft, occasionally fuel cell systems are conceived or already used in order to handle various tasks. Apart from electricity generation, other tasks can also be carried out, for example rendering a fuel tank inert by introducing the exhaust gases of a fuel cell system. Because of the way a fuel cell operates, the exhaust gas usually contains water vapor. Generally-speaking, if humid gases are used for rendering a fuel tank inert, there is a problem in that fuels, in particular kerosene, are hygroscopic. Furthermore, a bacteria population can form in the tank, which bacteria population could influence sensors for acquiring the fill level of the tank so that acquisition becomes imprecise. Furthermore, within the fuel tank or the fuel itself, ice crystals could form that could result in damage to engine injection nozzles and fuel filters in cruising flight of the aircraft or during below-zero temperatures on the ground. There is thus a requirement for introducing dry gases into the fuel tank in order to be able to render the fuel tank inert.
DE 10 2005 054 885 A1 and US 2007/0111060 A1 disclose a safety system for reducing the danger of explosion of a fuel tank, in which system a protective-gas production device comprises a fuel cell system with a fuel cell, and provides a protective gas which during operation of the fuel cell system is produced by the fuel cell.
In prior art various methods and systems are known that are used for drying gases, in particular air. Thus it would, for example, be possible to carry out adsorption with hygroscopic media, for example silica gel. However, the water absorption capacity of a hygroscopic medium is finite, and consequently after use it would have to either be replaced or regenerated. In particular in an aircraft, replacement leads to pronounced weight problems, and constant emptying and refilling leads to increased maintenance effort. Furthermore, regeneration would be possible with a corresponding heat input, for example, with heated air. However, this would place in doubt the effectiveness of the fuel cell system, because thermal regeneration would require considerable expenditure of energy. If no regeneration is to be carried out, due to the above-mentioned saturation, exhaust gas drying is possible only for a limited period of time. Generally speaking, in such methods dew points, i.e., temperatures, are attained at which there is a state of equilibrium between condensing water and evaporating water, which dew points or temperatures reach far into the double-digit negative region.
A further method for drying air takes place by water transfer with a selective membrane, with the use of a partial pressure differential. To this effect a membrane would be used that separates a gas to be dried from a dry gas, where, due to a partial pressure differential, water is made to pass through the membrane. As an alternative to the dry gas it would also be possible to increase the static pressure on that membrane side on which the gas to be dried is located. The drying performance of this method is limited by the achievable partial pressure differential. Particularly low dew points of a membrane compressed-air dryer are only achieved with the use of quite a high operating pressure and the accompanying high compressor performance necessary.
A further, third, method from prior art for gas drying would take place by cooling the gas to below the dew point, for which purpose basically only a heat exchanger and a heat sink or a cooling medium are required. Following cooling, and for final separation of liquid water from gaseous residual gas, a drip catcher or the like could be used. However, this principle requires quite considerable cooling capacity because liquid product water is present, and the energy released during the phase transition needs to be discharged. The cold used to cool the gas can in part be recovered in a downstream recuperative heat exchanger. Basically, in this arrangement the attainable dew point is limited by the freezing point, because in the design currently in widespread use icing occurring within the heat exchanger can result in the blocking of gas ducts.
Correspondingly, it may be considered at least one object to provide a system for cooling the exhaust gas of a fuel cell system, which system for cooling reduces or entirely eliminates the above-mentioned disadvantages. In particular, it may be considered at least another object to provide a system for drying exhaust gas of a fuel cell system, which system for drying with the use of as little energy as possible makes it possible to dry the exhaust gas as effectively as possible without significantly increasing the complexity of the fuel cell system or its periphery, while at the same time minimizing the additional weight. In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.